Gamma-secretase catalyzes the final cleavage that releases ABeta from its precursor; therefore, it is a potential therapeutic target for the treatment of AD. A great deal of evidence now supports the concept that presenilins (PS) are catalytic components of the high molecular weight gamma-secretase complex. We have identified a novel family of presenilin homologues (PSH). PSH can be aligned over most of their lengths with PS. Regions that are highly conserved between the two families include i) putative active site aspartate residues in TMD6 and TMD7, ii) TMD sequences in general, and iii) a 'PAL' motif that is reported to be obligatory for stabilization, complex formation, and gamma-secretase activities of PS. One of the PSH has now been shown to be an intramembrane cleaving protease. Based on this evidence we hypothesize that that PS and PSH represent a family of intramembraneous cleaving proteases with a conserved catalytic mechanism. As shown for other enzyme families, the identification of a wider repertoire of PS/PSH family members may now facilitate a greater understanding of the molecular and cellular functions of this biomedically-important family. Given the interest in developing gamma-secretase inhibitors that target PS as potential therapeutic agents, information gathered by studying this protein family may offer important insights into the potential catalytic mechanisms and substrate-specificities of PS and PSH that will aid the development of such inhibitors. These studies will also provide general insight as to how intramembrane cleavage of peptide bonds occurs. The questions we will address are 1) What are the basic characteristics and cellular roles of the PSH proteins? 2) Do PSs and PSHs utilize a catalytic mechanism similar to that known of other aspartyl proteases? 3) What are the structural similarities between the active sites of PSH and PS? Do PSHs possess an initial substrate docking site as PS apparently does? And 4) Does SPP/PSH3 indeed act alone (and as a dimer)?